Q: I just heard on the news that after a stroke or other traumatic brain injury, the damage to the white matter in the brain is a better predictor “of brain-function outcome” than gray matter. What does this mean?

A: The results of a study by researchers at the University of Iowa have challenged the commonly held idea that when it comes to brain health and brain function, gray matter (the neurons that form the cerebral or thinking cortex) in the left hemisphere is more important than white matter (the myelin covered axons that physically connect neuronal regions in the brain, especially in the right hemisphere.) This new study published in the Proceedings of the National Academy of Sciences in the US reported on the analysis of brain scans and cognitive or thinking-function tests from over 500 people with localized areas of brain damage caused by various forms of brain injury, including strokes. Looking at the location of the brain damage, also known as lesions, the research team correlated the level of connectedness of the damaged areas with the level of cognitive or thinking disability the patient experienced.

The first author on the study said they found that damage to gray matter hubs of the brain did not reveal much about how poorly people would do on cognitive  or thinking tests following a traumatic brain injury. The study also revealed that study participants with damage to the densest white matter connections did much worse on cognitive thinking tests. Apparently, this is a rather revolutionary finding since in the past, both scientists and clinicians have tended to focus almost exclusively on the role of gray matter in traumatic brain injuries including stroke. According to the researchers, the take-away from this study is that the connections between brain regions might matter as much if not more so in terms of cognitive thinking ability post brain trauma than the brain regions themselves.